Actually it was the other way round. I had just installed dmd 2.057 and the matching gtkd 1.5.1 from the .deb mentioned in the forum.

The latter installed the library in /usr/lib/gtkd/32. But 'Search for files' showed only one libgtkd.a in /usr/local/lib quite reproducibly, and it was that - an older version of libgtkd that I was linking against.